JP2012062515A - Bearing steel excellent in cold workability, wear resistance and rolling fatigue characteristics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide bearing steel enabling to exert favorable cold workability during cold working that is performed after globulariizing annealing, and also enabling to secure favorable wear resistance and rolling fatigue characteristics as a bearing member or the like.SOLUTION: The bearing steel contains 0.9-1.10% C, 0.05-0.49% Si, 0.1-1.0% Mn, 0.05% or less P (excluding 0%), 0.05% or less S (excluding 0%), 0.03-0.40% Cr, 0.05% or less Al (excluding 0%), 0.002-0.025% N, 0.0030% or less Ti (excluding 0%), and 0.0025% or less O (excluding 0%), the remainder comprising iron and unavoidable impurities. In the bearing steel, the average aspect ratio of cementite is 2.00 or less, the average circle equivalent diameter of cementite is 0.35-0.6 μm, and the number density of cementite having a circle equivalent diameter of 0.13 μm or more is 0.45/μmor more.

Description

  The present invention relates to a bearing steel applied to bearing parts and machine structural parts used in automobiles, various industrial machines, and the like, and particularly when manufacturing bearing parts and machine structural parts by cold working. The present invention relates to a steel for bearings that exhibits good cold workability and exhibits excellent wear resistance and rolling fatigue characteristics in the processed parts.

  Bearing parts and machine structural parts are processed into final shapes by performing processing such as cutting, forging, and cutting of wire rods and steel bars. Especially for cold working (cold rolling or cold forging), the rolled material is too hard and cold working is difficult, so spheroidization prior to cold working for the purpose of improving cold workability. In general, annealing is performed.

Ensuring good cold workability is important from the viewpoint of improving productivity and energy saving, and reducing costs and reducing CO ₂ emissions. In order to ensure good cold workability, it is necessary to have low deformation resistance, no cracking due to processing, and the like.

  Parts such as bearings and crankshafts are important parts that support the rotating parts and sliding parts of machinery, and the contact surface pressure is considerably high, and the external force may fluctuate. In many cases, the steel material is required to have excellent durability.

  In recent years, these requirements have become stricter year by year as the performance and weight of machinery have been improved. In order to improve the durability of shaft parts, it is important to improve the technology related to lubrication, but it is particularly important that the steel material has excellent wear resistance and rolling fatigue characteristics.

  As steel materials used for bearings, conventionally, high carbon chromium bearing steel such as SUJ2 defined in JIS G 4805 (1999) has been used in various fields such as automobiles and various industrial machines. However, this steel has a high content of carbon (0.95 to 1.1% by mass) and Cr (1.3 to 1.6% by mass), which is a coarse eutectic carbide that adversely affects rolling fatigue characteristics ( For example, a precipitate of 10 μm or more is easily generated. In order to prevent the formation of this coarse eutectic carbide, it is necessary to perform rolling after high-temperature (about 1250 ° C.) and long-time (about 17 hours) diffusion rolling, resulting in high production costs. It was. Moreover, the bearing component manufactured with this steel material was known to have a problem of insufficient rolling fatigue characteristics.

  Under such circumstances, various technologies have been proposed as bearing steels.

  For example, in Patent Document 1, the content of C (0.6 to less than 0.95 mass%) and Cr (less than 1.3 mass%) is reduced, and B (0.0002 to 0.01 mass%) is reduced. By including a predetermined amount, a technique has been proposed in which the homogenization heat treatment is omitted and the spheroidizing annealing time is reduced, thereby reducing the manufacturing cost and ensuring excellent rolling fatigue characteristics and wear resistance. However, with this technique, consideration is not given to cold forgeability, and problems such as cracking may occur during cold forging.

  On the other hand, Patent Document 2 contains a predetermined amount of Sb (less than 0.0010 mass%) while reducing the content of C (0.70 to 0.95 mass%) to shorten the diffusion annealing time. Thus, a technique for improving rolling fatigue characteristics has been proposed. However, even with this technique, consideration is not given to cold forgeability, and problems such as cracking may occur during cold forging.

  Patent Document 3 discloses a patent in which the average grain size of ferrite and the average grain size of cementite after cold drawing are defined after spheroidizing annealing treatment to improve cold workability. However, since the content of C and Cr is large and eutectic carbide may be generated, diffusion annealing is essential, and after further spheroidizing annealing, cold drawing is performed at 20 to 40%. , The yield of steel materials deteriorates and the manufacturing cost increases.

JP 2000-96185 A JP-A-10-158790 JP 2001-294972 A

  The present invention has been made paying attention to the circumstances as described above, and the purpose thereof is to exhibit good cold workability in cold working performed after spheroidizing annealing, and to provide a bearing. The purpose is to establish a bearing steel that can ensure good wear resistance and rolling fatigue characteristics as a member. Still another object of the present invention is to provide a steel material that can produce a steel for bearings having such excellent characteristics even if the diffusion annealing is omitted.

The steel for bearings of the present invention that can achieve the above-mentioned object is: C: 0.9 to 1.10% (meaning of mass%, the same applies hereinafter), Si: 0.05 to 0.49%, Mn: 0.00. 1 to 1.0%, P: 0.05% or less (not including 0%), S: 0.05% or less (not including 0%), Cr: 0.03 to 0.40%, Al: 0.05% or less (not including 0%), N: 0.002 to 0.025%, Ti: 0.0030% or less (not including 0%), and O: 0.0025% or less (0% The balance is composed of iron and inevitable impurities, the average aspect ratio of cementite is 2.00 or less, the average equivalent circle diameter of cementite is 0.35 to 0.6 μm, and the equivalent circle diameter is 0 It has a gist in that the number density of cementite of 13 μm or more is 0.45 pieces / μm ² or more. .

  In the present invention, as other elements, Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), and Mo: 0.25% or less (0%) It is also preferable to include one or more selected from the group consisting of Nb: 0.5% or less (not including 0%), and / or V: 0.5. % Or less (not including 0%) is also a preferred embodiment.

According to the present invention, by appropriately adjusting the chemical composition and appropriately dispersing (number density) cementite having an appropriate size (aspect ratio, equivalent circle diameter) in the steel material, good cold working is achieved. It is possible to realize a bearing steel that can secure excellent wear resistance and rolling fatigue characteristics (hereinafter, sometimes referred to as “durability” together with rolling fatigue characteristics and wear resistance). Therefore, when the bearing steel of the present invention is applied to a bearing component, excellent durability can be exhibited even when used in a harsh environment. Further, the bearing steel of the present invention with reduced Cr does not require long-time diffusion annealing, which has been essential in conventional SUJ2, and can also perform spheroidizing annealing in a short time, so that productivity can be improved. It is also useful from the viewpoint of improvement and energy saving, cost reduction, and CO ₂ emission reduction.

FIG. It is the photograph of the coarse carbide | carbonized_material confirmed in the steel piece of 2. FIG. FIG. 2 is a graph plotting the relationship between the average equivalent circle diameter of cementite, the deformation resistance reduction rate, and the L10 life ratio. FIG. 3 is a graph plotting the relationship between the number density of cementite and the wear ratio.

The present inventors have studied from various angles with the aim of realizing a bearing steel that exhibits excellent cold workability and durability. And, in order to improve the cold workability of the steel material and improve the durability, it is necessary to appropriately control the chemical composition of the steel material and to control the cementite, particularly the following (A) and (B). The knowledge that satisfying the requirements was effective was obtained.
(A) In order to improve cold workability and rolling fatigue characteristics, the size of cementite (average equivalent circle diameter and average aspect ratio) should be controlled within a predetermined range. (B) To improve wear resistance, Control the number density of cementite within a specified range.

Based on the above findings, the present inventors have made extensive studies to improve the cold workability, wear resistance, and rolling fatigue characteristics of steel materials. As a result, the chemical component composition in the steel material is controlled and the production conditions are controlled. After spheroidizing annealing, the average aspect ratio of cementite is 2.00 or less, and the average equivalent circle diameter of cementite is 0.35 to 0.6 μm. Thus, the present inventors have found that the above properties of steel can be improved when the number density of cementite having an equivalent circle diameter of 0.13 μm or more is 0.45 / μm ² or more, and the present invention has been completed.

  In the present invention, the formation of coarse eutectoid carbide is suppressed by reducing the Cr content. However, when the Cr content is reduced, it becomes difficult to obtain a steel that satisfies the above-mentioned prescribed cementite. Therefore, as a result of repeated research on manufacturing conditions, even if the Cr content is reduced, the above-defined cementite can be satisfied by strictly controlling the manufacturing conditions, and it has been essential for conventional steel (SUJ2). It has been found that the steel for bearings of the present invention having the above excellent characteristics can be produced even if the time-diffusion annealing is omitted and the spheroidizing annealing time is further shortened.

  First, the cementite specified by the present invention will be described.

  In the present invention, the average aspect ratio of cementite needs to be 2.00 or less. When this average aspect ratio is greater than 2.00, stress tends to concentrate on the cementite during cold working, cracks are easily generated at the interface, and the rolling fatigue characteristics are also deteriorated. The average aspect ratio is preferably 1.90 or less, more preferably 1.70 or less. The aspect ratio is a ratio of the major axis and the minor axis of cementite, and is based on the measurement method described in the examples below. The average aspect ratio of the present invention is an average value of 12 visual fields.

  In the present invention, since the average size of cementite also affects cold workability and rolling fatigue characteristics, the average equivalent circle diameter needs to be 0.35 to 0.6 μm. When the average equivalent-circle diameter of cementite is less than 0.35 μm, deformation resistance increases due to dispersion strengthening, and cold workability deteriorates. On the other hand, when the thickness is less than 0.35 μm, the cementite disappears due to the quenching / tempering treatment, and the desired durability cannot be obtained. A preferable average equivalent circle diameter is 0.40 μm or more, more preferably 0.45 μm or more. On the other hand, when the average equivalent circle diameter of cementite exceeds 0.6 μm, the fragile portion around the cementite after quenching and tempering becomes large, so that cracks are easily generated and propagated, resulting in poor rolling fatigue characteristics. The average equivalent circle diameter is preferably 0.55 μm or less, more preferably 0.5 μm or less.

  The above-mentioned equivalent circle diameter refers to the diameter of a circle that is assumed to have the same area by paying attention to the size of cementite, and will be described later in Examples, but it is described in the scanning electron microscope (SEM). It is a cementite observed on the observation surface, and the average equivalent circle diameter of the present invention is an average value of 12 visual fields.

Further, in the present invention, when the number density of cementite having an equivalent circle diameter of 0.13 μm or more confirmed when observing the above cementite is less than 0.45 / μm ² , the effect of improving the wear resistance by dispersion of hard cementite. Will not be effective. Accordingly, the number density of cementite having an equivalent circle diameter of 0.13 μm or more is preferably 0.48 pieces / μm ² or more, more preferably 0.51 pieces / μm ² or more. The upper limit of the number density is not particularly limited, but if it is too large, deformation resistance may increase due to dispersion strengthening, and cold workability may deteriorate. The number density of cementite is preferably 1.0 piece / μm ² or less, more preferably 0.75 piece / μm ² or less. The number density of the present invention is a value obtained by observing 12 fields of view, which will be described later in Examples.

  In the observation of the cementite, coarse precipitates are excluded from the observation. The coarse precipitate means, for example, one having a major axis of 10 μm or more.

  Each value of the above cementite is measured based on the method described in the examples. Although described in the examples, the present invention adopts a value obtained by observing the D / 4 position (D is a diameter) of the steel material. This is because, if the measurement result at the D / 4 position satisfies the above-mentioned provisions of the present invention, it exhibits not only cold workability but also excellent wear resistance and rolling fatigue characteristics of the parts after processing. .

  The steel material of the present invention needs to appropriately adjust its chemical composition (C, Si, Mn, P, S, Cr, Al, N, Ti, O, etc.) including the reduction of Cr content. The reasons for limiting the ranges of these components are as follows.

[C: 0.9 to 1.10%]
C is an element essential for increasing quenching hardness, maintaining strength at room temperature and high temperature, dispersing cementite to impart wear resistance and rolling fatigue characteristics, and improving cold workability It is. In order to exert such an effect, C must be contained in an amount of 0.9% or more, preferably 0.95% or more, more preferably 0.97% or more. However, if the C content is excessively large, giant carbides are likely to be formed in the core, and adversely affect the rolling fatigue characteristics. Therefore, the C content is 1.10% or less, preferably 1.07. % Or less, more preferably 1.03% or less.

[Si: 0.05 to 0.49%]
Si is an element useful for improving the solid solution strengthening and hardenability of the matrix. In order to exert such effects, it is necessary to contain Si by 0.05% or more, preferably 0.1% or more, and more preferably 0.2% or more. However, if the Si content is excessively increased, the cold workability is remarkably reduced, so the Si content should be suppressed to 0.49% or less, preferably 0.35% or less, more preferably 0.30% or less. is there.

[Mn: 0.1 to 1.0%]
Mn is an element useful for strengthening the solid solution of the matrix and improving the hardenability. In order to exert such an effect, it is necessary to contain Mn in an amount of 0.1% or more, preferably 0.15% or more, more preferably 0.2% or more. However, if the Mn content is excessively increased, the cold workability is remarkably reduced, so the Mn content should be suppressed to 1.0% or less, preferably 0.85% or less, more preferably 0.8% or less. is there.

[P: 0.05% or less (excluding 0%)]
P is an element inevitably contained as an impurity, but it is desirable to reduce it as much as possible because it segregates at the grain boundary and lowers the cold workability. However, extreme reduction leads to an increase in steelmaking cost. become. For these reasons, the P content is set to 0.05% or less. It is preferable to reduce it to 0.04% or less, more preferably 0.03% or less.

[S: 0.05% or less (excluding 0%)]
S is an element that is inevitably contained as an impurity, but is precipitated at the grain boundary as FeS and decreases the cold workability. Moreover, although it is desirable to reduce as much as possible in order to precipitate as MnS and to reduce a rolling fatigue characteristic, extreme reduction will cause the steelmaking cost to increase. For these reasons, the S content is set to 0.05% or less. It is preferable to reduce it to 0.04% or less, more preferably 0.03% or less.

[Cr: 0.03-0.40%]
Cr is an element that combines with C to form carbides and improves wear resistance and cold workability. In order to obtain such an effect, the Cr content needs to be 0.03% or more. Preferably it is 0.1% or more, More preferably, it is 0.2% or more. However, since Cr is an element that is easily segregated, if the Cr content is excessive, coarse carbides are generated, and the rolling fatigue characteristics are lowered. Therefore, the Cr content is 0.40% or less. Preferably it is 0.35% or less, More preferably, it is 0.3% or less.

[Al: 0.05% or less (excluding 0%)]
Al is effective as a deoxidizing element, has an effect of reducing the amount of oxygen in steel and improving rolling fatigue characteristics, and is an element that combines with N to form AlN and improve rolling fatigue characteristics. It is. In order to obtain such an effect, it is desirable to contain 0.015% or more of Al. However, when the amount of Al is excessive, alumina inclusions are coarsened and rolling fatigue characteristics are deteriorated. Therefore, the Al content is 0.05% or less, preferably 0.04% or less, more preferably 0.03% or less.

[N: 0.002 to 0.025%]
N is an element that combines with the Al and exhibits an effect of improving rolling fatigue characteristics by fine dispersion of an Al-based nitrogen compound. In order to exert such an effect, the N content is 0.002% or more, preferably 0.004% or more, more preferably 0.005% or more. However, when the N content is excessive, coarse TiN is formed, and rolling fatigue characteristics are deteriorated. Therefore, the N content is 0.025% or less, preferably 0.020% or less, more preferably 0.010% or less.

[Ti: 0.0030% or less (excluding 0%)]
Ti combines with N in steel to produce TiN, which not only adversely affects rolling fatigue properties but also harms cold workability and hot workability, and it is desirable to reduce it as much as possible. However, an extreme reduction leads to an increase in steelmaking costs. For these reasons, the Ti content needs to be 0.0030% or less. In addition, the upper limit with preferable Ti content is 0.0015% or less, More preferably, it is 0.0010% or less.

[O: 0.0025% or less (excluding 0%)]
O has a large effect on the form of impurities in steel and forms inclusions such as Al ₂ O ₃ and SiO ₂ that adversely affect rolling fatigue characteristics. Doing so will increase the steelmaking cost. For these reasons, the O content needs to be 0.0025% or less. In addition, the upper limit with preferable O content is 0.002% or less, More preferably, it is 0.0015% or less.

  The contained elements specified in the present invention are as described above, and the balance is iron and unavoidable impurities, and as the unavoidable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed. In addition, in order to improve rolling fatigue characteristics, it is also possible to positively contain the following elements within a specified range.

[From the group consisting of Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), and Mo: 0.25% or less (not including 0%) Including one or more selected]
Cu, Ni, and Mo all act as elements for improving the hardenability of the parent phase, increase the hardness and contribute to the improvement of rolling fatigue characteristics, and can contain any one or more kinds. . Any of these effects is preferably exhibited by containing 0.03% or more, more preferably 0.05% or more. However, if both exceed 0.25%, workability deteriorates. Preferably it is 0.23% or less, More preferably, it is 0.20% or less.

[Nb: 0.5% or less (not including 0%) and / or V: 0.5% or less (not including 0%)]
Nb and V are elements that are effective in binding nitrogen to form a nitrogen compound to regulate crystal grains and improve rolling fatigue characteristics, and can be used alone or in combination. . All of these effects are preferably exhibited by containing 0.001% or more, more preferably 0.003% or more. However, if any content exceeds 0.5%, the crystal grains become finer and an incompletely quenched phase tends to be generated. The preferred contents are each 0.3% or less, more preferably 0.1% or less.

  In this invention, although Cr content is prescribed | regulated in the said range, when Cr content is reduced to 0.40% or less, it will become difficult to disperse | distribute the said predetermined magnitude | size cementite stably. Therefore, in the steel material of the present invention, in order to secure cementite having a predetermined size after spheroidizing annealing, it is necessary to appropriately control the manufacturing conditions (especially spheroidizing annealing conditions performed after rolling). In the present invention, the steel material obtained by hot rolling is heated to a soaking temperature (A1 point + 10 ° C. to A1 point + 40 ° C.) at an average heating rate of 40 to 100 ° C./hr, and the soaking temperature is 4 to 4 ° C. After holding for 8 hours, from the soaking temperature to (A1 point-60 ° C), the primary cooling rate (average cooling rate) is in the range of 5-15 ° C / hr, and further up to room temperature (25 ° C). By letting the next cooling stand to cool, the cementite having the predetermined size can be stably dispersed.

  When the average heating rate is less than 40 ° C./hr, the cementite is coarsened, and the cementite having the desired average equivalent circle diameter cannot be obtained, and a dispersion state of the cementite (that is, a predetermined number density) is obtained. Disappear. When it exceeds 100 ° C./hr, the pearlite cannot be divided, and the aspect ratio of cementite exceeds the predetermined value. On the other hand, if it exceeds 100 ° C./hr, the cementite becomes small, and the cementite having the predetermined average equivalent circle diameter cannot be obtained. A preferable average heating rate is 50 ° C./hr or more, more preferably 60 ° C./hr or more, preferably 90 ° C./hr or less, more preferably 80 ° C./hr or less.

  If the soaking temperature is less than A1 point + 10 ° C., pearlite cannot be divided, and the aspect ratio of cementite exceeds the predetermined value. On the other hand, when the soaking temperature exceeds the point A1 + 40 ° C., cementite in the pearlite is excessively dissolved and the regenerated pearlite is precipitated, so that the aspect ratio exceeds the predetermined value.

  When the holding time at the soaking temperature (A1 point + 10 ° C. to A1 point + 40 ° C.) is less than 4 hours, the cementite diameter becomes small and the predetermined size cannot be obtained. On the other hand, when the holding time exceeds 8 hours, the cementite is coarsened, the cementite diameter is increased, the predetermined size cannot be obtained, and the dispersed state of cementite cannot be obtained.

  When the primary cooling rate is less than 5 ° C./hr, cementite is coarsened and a desired size cannot be obtained. On the other hand, if it exceeds 15 ° C./hr, the cementite diameter becomes small and the predetermined size cannot be obtained.

  The primary cooling is in the range from the soaking temperature to (A1 point-60 ° C.), which is a temperature set in order to surely obtain the prescribed cementite. Therefore, as long as the prescribed cementite is obtained, the temporary cooling end temperature may be high (for example, A1 point-50 ° C.). Moreover, if it exceeds A1 point-60 degreeC and it cools with the said primary cooling rate, productivity will deteriorate.

  In addition, the secondary cooling rate is not particularly limited, but it is desirable to cool it (air cooling) from the viewpoint of improving productivity.

  The steel material of the present invention is processed into a predetermined part shape after the spheroidizing annealing as described above, and is subsequently quenched and tempered to be manufactured into a bearing part or the like. It includes both linear and bar shapes that can be applied to such production, and the size can also be appropriately determined according to the final product.

  The bearing steel of the present invention defines a structure after spheroidizing annealing, but a bearing steel that satisfies the above defined structure after spheroidizing annealing is processed into a predetermined part shape. It has been confirmed by experiments that it exhibits excellent wear resistance and rolling fatigue characteristics when used as a bearing part that is subsequently quenched and tempered (see Examples below).

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steel materials having various chemical composition compositions shown in Table 1 below were heated to 1100 to 1300 ° C. in a heating furnace, and then subjected to ingot rolling at 900 to 1200 ° C. Thereafter, hot rolling (including forging simulating rolling) was performed in a temperature range of 800 to 1100 ° C. to produce a round bar with a diameter of 65 mm.

  The obtained round bar was subjected to spheroidizing annealing under the spheroidizing heat treatment conditions shown in Table 2 (temperature increase rate, soaking temperature, soaking time, primary cooling rate to Al point -60 ° C) and tested. Got. Under the present circumstances, from A1 point -60 degreeC to room temperature (25 degreeC), it air-cooled (secondary cooling).

  In addition, No. After carrying out decomposition rolling only for No. 1, hot rolling was performed after performing soaking (diffusion annealing) at 1230 ° C. for 17 hours in a soaking furnace (No. 1 was used as a reference steel for comparison).

[With or without giant carbides]
Regarding the D / 2 position (D is the diameter) of each test material, the cross-sectional macrostructure was observed with an optical microscope (magnification: 100 times) (number of observations: 1 field of view) to confirm the presence or absence of giant carbide. When giant carbides of 10 μm or more as shown in the photograph of FIG. 1 could be confirmed, it was determined that the giant carbides were “present”.

[Number density of cementite, average equivalent circle diameter, average aspect ratio]
The test material was cut perpendicularly to the longitudinal direction (rolling direction), and the test material was further cut horizontally in the longitudinal direction at the D / 4 position (D is the diameter) of this longitudinal section (perpendicular to the rolling direction). The horizontal cut surface was mirror-polished and corroded with 5% picral to reveal the metal structure, and then any 12 locations on the D / 4 position line on this horizontal cut surface (2688 μm ² per field of view) Were observed and photographed under a scanning electron microscopic environment (magnification: 2000 times), and white portions were identified as cementite particles from the contrast of the image and marked. Using particle analysis software ([Particle Analysis III for windows. Version 3.00 SUMITOMO METAL TECHNOLOGY]), the equivalent circle diameter (μm) is calculated from the area of each marked cementite particle, and the average value of 12 fields of view is obtained. ("Average equivalent circle diameter").

In addition, the number (number / μm ² ) of cementite particles having an equivalent circle diameter of 0.13 μm or more per unit area was determined (“number density”).

  Further, the aspect ratio of cementite was determined, and the average value of 12 fields of view was determined (“average aspect ratio”).

  In the measurement of cementite, those with a size of less than 0.13 μm were excluded from measurement.

[Cold workability (cracking / deformation resistance)]
Using the above test material, a cylindrical test piece having a diameter of 14 mm and a height of 21 mm was cut out from the center of the test material, and used as a test piece for evaluating cold workability.

The test piece was cold worked using a press tester at a processing rate (compression rate) of 60%, and then the side surface of the test piece was observed with an optical microscope (magnification: 20 times) to confirm the presence or absence of cracks. Further, the deformation resistance (MPa) when the test piece was cold worked at a working rate (compression rate) of 40% was measured. The deformation resistance reduction rate was calculated by comparison with the deformation resistance of the test piece 1 and evaluated. The above working ratio is represented by [{(1-L / L 0)} × 100 (%)] (L is the length of the specimen before the processing, the length of L ₀ is after working specimen) Is.

  Evaluation criteria: The test piece is not cracked and the deformation resistance reduction rate is No. It was judged that 5% or more of one test piece was excellent in cold workability (◯). On the other hand, the deformation resistance is No. Those having a deformation resistance reduction rate of less than 5% were judged to be inferior in cold workability (×) even when the deformation resistance was higher than 1 or the deformation resistance was reduced.

[Rolling fatigue characteristics]
A thrust test piece (shape: disk size: Φ60 mm × 2 mm thickness) is prepared from the test material, and the repetition rate is 1800 rpm with a thrust type rolling fatigue tester (“FJ-5T” manufactured by Fuji Testing Machine Co., Ltd.). , Surface pressure: 5.3 GPa, number of discontinuations: 2 × 10 ⁸ rolling fatigue tests were performed 16 times for each test piece, fatigue life L ₁₀ (obtained by plotting on wipe probability paper) The number of repeated stresses until fatigue failure at a cumulative failure probability of 10% was evaluated.

No. The fatigue life L ₁₀ (L10 life) of the test piece 1 was 1.0, and a test piece having an L10 life of 1.0 or more was judged to be excellent in rolling fatigue characteristics.

[Abrasion resistance]
Wear depth when the thrust test piece is rotated on a thrust type rolling fatigue tester under the conditions of repetition rate: 1800 rpm, surface pressure: 5.3 Gpa, number of interruptions: 1 × 10 ⁸ times Was the amount of wear. At this time, the number of tests for each steel material was three (n = 3). No. The amount of wear of one test piece was set to 1, and a test piece having a wear amount of 1.00 or less was judged to be excellent in wear resistance.

  From these results, it can be considered as follows. That is, no. 3-6, 8, 9, 12, 13, 16, 17, 20, 21, 23, 24, 27-29 are the requirements defined in the present invention (chemical composition, cementite equivalent circle diameter, aspect ratio, number Density), and without any cracks, the deformation resistance is low compared to the conventional steel (No. 1: SUJ2), and the cold workability is excellent. Excellent fatigue properties and wear resistance.

  No. In No. 2, since the Cr content was large, giant carbide was generated in the steel piece, and cold workability, rolling fatigue characteristics, and wear resistance were inferior, and the test piece was cracked.

  No. 7, 10, 11, 14, 15, 18, 19, and 22 are examples outside the range of the spheroidizing heat treatment condition defined in the present invention.

  No. No. 7 has a slow temperature rise rate, so that the average equivalent-circle diameter and number density of cementite are outside the range defined in the present invention, and the rolling fatigue characteristics and wear resistance are inferior.

  No. No. 10, since the temperature rise rate is fast, the average equivalent circle diameter and average aspect ratio of cementite are outside the range defined in the present invention, and cold workability, rolling fatigue characteristics, and wear resistance are inferior. At the same time, the test piece was cracked.

  No. No. 11 has a low soaking temperature, the average aspect ratio of cementite exceeds the range specified in the present invention, and the cold workability, rolling fatigue characteristics, and wear resistance are inferior. Cracking occurred.

  No. No. 14, because the soaking temperature is high, the average aspect ratio of cementite exceeds the range specified in the present invention, the rolling fatigue characteristics and the wear resistance are inferior, and the test piece was cracked. .

  No. No. 15, because the soaking time is short, the average equivalent circle diameter of cementite is below the range defined in the present invention, and the cold workability is inferior.

  No. In No. 18, since the soaking time is long, the average equivalent circle diameter and number density of cementite are outside the range defined in the present invention, and the rolling fatigue characteristics and the wear resistance are inferior.

  No. No. 19 has a slow primary cooling rate (cooling rate up to A1 point −60 ° C.), so the average equivalent circle diameter of cementite exceeds the range defined in the present invention, and the rolling fatigue characteristics are inferior.

  No. Since No. 22 has a high primary cooling rate, the average equivalent-circle diameter of cementite is below the range defined in the present invention, and the cold workability is poor.

  No. 25, 26, 30 to 31 are examples out of the range of chemical components defined in the present invention.

  No. No. 25 has a low C content, so that the number density of cementite is insufficient and the wear resistance is inferior.

  No. No. 26, Si, Mn, P, S, Al, Ti, O are outside the range specified in the present invention, the number density of cementite becomes insufficient, cold workability and rolling fatigue characteristics are inferior Yes.

  No. No. 30 has a large Cr and O content, so that the average equivalent-circle diameter of cementite is less than that of the present invention, and a giant carbide is generated in the steel slab, resulting in poor rolling fatigue characteristics.

  No. No. 31 is inferior in rolling fatigue characteristics because the Si, Mn, N, P, and S contents are outside the range defined in the present invention.

  No. No. 32 is inferior in cold workability and rolling fatigue characteristics because Cr and N are outside the range defined in the present invention.

  Based on these data, the relationship between the average equivalent circle diameter of cementite, the deformation resistance reduction rate (cold workability), and the L10 life ratio (rolling fatigue properties) is shown in FIG. 2 (the chemical composition defined in the present invention). It is shown in the plot) that the average aspect ratio is less than or equal to 2.00. However, appropriately controlling the cementite size is effective in improving cold workability and rolling fatigue characteristics. I understand.

  Similarly, the relationship between the number density of cementite and the wear ratio (abrasion resistance) is shown in FIG. 3 (only the example that satisfies the chemical composition defined in the present invention and has an aspect ratio of 2.00 or less is plotted). It can be seen that appropriately controlling the number density of cementite is effective in improving the wear resistance.

Claims (3)

C: 0.9 to 1.10% (meaning mass%, the same shall apply hereinafter)
Si: 0.05-0.49%
Mn: 0.1 to 1.0%,
P: 0.05% or less (excluding 0%),
S: 0.05% or less (excluding 0%),
Cr: 0.03 to 0.40%,
Al: 0.05% or less (excluding 0%),
N: 0.002 to 0.025%,
Ti: 0.0030% or less (not including 0%), and O: 0.0025% or less (not including 0%), the balance is made of iron and inevitable impurities, and the average aspect ratio of cementite is 2 0.003 or less, the average equivalent circle diameter of cementite is 0.35 to 0.6 μm, and the number density of cementite having an equivalent circle diameter of 0.13 μm or more is 0.45 / μm ² or more. Bearing steel with excellent cold workability, wear resistance and rolling fatigue characteristics.   Further, as other elements, Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), and Mo: 0.25% or less (not including 0%) The bearing steel according to claim 1, comprising one or more selected from the group consisting of:   The bearing according to claim 1 or 2, further comprising Nb: 0.5% or less (not including 0%) and / or V: 0.5% or less (not including 0%) as other elements. steel.